There are few forms of gambling that offer a positive expectancy attainable through skill and diligence. Poker is probably a notable exception.

Speculation and investing do through market inefficiencies and risk premia respectively.

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I agree with this. There are mathematical proofs that certain gambling games will never results in positive expectancy, but there are trading methods which can show positive expectancy over hundreds or thousands of trades.

The main problem with the OP's question, of course, is that you actually can't explain that to a 5 year old.

It has pretty much been shown, to the limits of being able to be proven, that successful trading actually is a skill.

Again, though, these are not analyses that can easily be explained to 5-year olds. In fact, it appears that many reasonably intelligent adults are incapable of following the mathematics and statistics behind the conclusions.

Casino cheats are thrown out, sometimes jailed. Market cheats are rewarded, given incentive and financed with tax payer dollars to run the scam all over again. Casino's have government oversight, rules and regulations. Current market conditions offer little of that, except for the retail trader. Lot's of rules and reg's for the little guy. Pretty much a free for all for the big boys.
So it pretty much depends on who is doing the trading. If you're one of the chosen few the fix is in and you'll make money no matter what. Any potential losses will be socialized among the masses and you'll be financed to gamble some more. At the end of the day the only people "trading" are retail. GS doesn't trade anything. They front OPM for a guaranteed return, one way or the other.